A molecular switch of the type III secretion system in Chlamydia trachomatis The obligate intracellular bacterium C. trachomatis is the causative agent of the most common sexually transmitted disease worldwide and represents a significant public health burden. The severe sequelae of genital chlamydial infections in women include pelvic inflammatory disease, ectopic pregnancy and tubal infertility. A key virulence mechanism of C. trachomatis is the type III secretion system (T3SS) that directly delivers protein effectors into the host cell cytosol to subvert host immunity and enables bacterial survival in hosts. The objective of this project is to study the mechanism by which C. trachomatis controls type III secretion and gene expression, contributing to disease pathogenesis. We hypothesize that T3SS activity is regulated and coupled to gene transcription during C. trachomatis infection by a molecular switch, consisting of CT663 and its protein partners. This hypothesis is strongly supported by our novel finding that chlamydial CT663 is a bi-functional protein acting as both a transcription regulator that interacts with RNA polymerase containing ? 66 and a T3SS chaperone for CopN, which is a regulator as well as an effector of the T3SS. Our specific aims are: Aim 1. To test the hypothesis that CT663 forms a protein complex necessary for type III secretion activity. We will characterize the dynamic interactions of CT663 and its partners, and how these interactions impact the secretion activity using quantitative assays for protein-protein interactions, immunodetection, and an enteropathogenic Escherichia coli (EPEC) system. Aim 2. To test the hypothesis that CT663 differentially regulates the gene transcription. This aim will be achieved using our established transcription assays with reconstituted ?66RNA polymerase in vitro and in E. coli. These studies will uncover how T3SS activity affects gene expression during C. trachomatis infection and vice versa. This project will provide important insights into how C. trachomatis utilizes T3SS to survive in an intracellular niche by coordinating the regulatory events of the T3SS and gene expression during infection. Our research will significantly expand current knowledge of the C. trachomatis infection process and contribute to the identification of potential drug targets for new therapies that could significantly reduce the public health burden caused by C. trachomatis infection.